It has been used as a standard unit of weight measurement since the time of the French revolution, based on the precise mass of a lump of metal held in Paris.

But the kilogram may be about to be given a new, and more precise, definition that for the first time will not require reference to a physical object.

Metrologists – scientists who study weights and measures – have developed a method of describing the kilogram by measuring the precise number of atoms in a piece of silicon.

This golf-balled sized cylinder of platinum and iridium shown above has been used for more than a century as the official definition of a kilogram, but it may now be replaced after scientists have cracked a way of calculating the number of atoms in a kilogram of silicon, which could remove the need for a physical reference

They hope it will overcome the gradual change in the mass of the International Prototype Kilogram that is currently used as the ultimate reference point worldwide for the unit of measurement.

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Over time tiny amounts of the metal – 90 per cent platinum and 10 per cent iridium – has been lost from corrosion and handling of the cylinder as it is removed from its safe.

WHY THE KILO IS LOSING WEIGHT

Kilograms of different materials are based on the measurement of the International Prototype Kilogram (IPK).

According to the Bureau International des Poids et Mesures (BIPM): 'The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.'

The cylinder, which is meant to weigh exactly a kilogram, has a diameter of 3.9cm made from an alloy that is 90 per cent platinum and 10 per cent iridium.

It has been looked after in Paris by the BIPM since 1889 and replacements have been made to maintain its accuracy.

The unit of mass is disseminated throughout the world via 'official copies'.

However, the official IPK has shed 100 micrograms (0.0001g) in the last century, prompting experts to find a more exact way of defining the kilogram.

There are fears that the IPK could also be destroyed or stolen, meaning the offical definition would be lost for all time.

The official kilogram weight lost 0.0001g, which is around the weight of a dust particle, in the last century, meaning the official weight is becoming less accurate.

The researchers have now found they can produce a reliable and accurate definition of the kilogram that does not change by calculating it in mathematical terms.

Dr Giovanni Mana, a researcher from the National Institute for Metrology Research in Turin who led the work, said the new definition of the kilogram could help to spur on new scientific research.

He said: 'The absence of technologies to redefine the kilogram is the biggest impediment to a redefinition of the whole system of measurement units, which is expected to deliver even more solid foundations and reliability to precision measurements and to set the stage for further innovations in technology and science.'

Dr Mana and his colleagues developed the new definition by calculating a mathematical constant called Avogadro's number – the number atoms or molecules in a certain volume of a substance.

Avogadro’s number is 'unfathomably large' – 6.022x10^23 which is greater than the number of grains of sand on earth or the stars in the universe.

This allows the number of silicon atoms in a kilogram to be counted within an accuracy of 20 atoms per billion, plus or minus, by defining it in terms of Avogadro's number.

All kilograms in the world, including those measured by these bathroom scales, are based on the official international prototype held at the International Bureau of Weights and Measures in Sevres

This can then be linked to another mathematical constant - called Planck's constant – which is used to describe the behaviour of quantum particles.

The two measures should agree with each other.

The new definition, if accepted, could allow scientists to change how they define a kilogram after decades of work.

First proposed in 1795, the kilogram was initially measured using the weight of water in a cube measuring 10 cubic cm at the melting point of water.

This was replaced in 1799 by a piece platinum before then being defined as the mass of a cylinder 39.17mm high made of 90 per cent platinum and 10 per cent iridium.

All other kilograms have been based on this golf-ball sized piece of metal, which is kept at the International Bureau of Weights and Measures in Sevres, near Paris.

Scientists around the world have been working to find ways to redefine the kilogram. Above a scientist holds a 1kg silicon sphere being used for the project

Dr Mana, whose research is published in the Journal of Physical and Chemical Reference Data, said the new definition would finally free scientists from having to use a physical object controlled by the French authorities.

He said: 'In metrology, it is important to ensure independence and democracy, to avoid the monopoly of a single nation or laboratory.'

The kilogram is currently the only standard measurement unit, known as a SI, to be based on a physical artefact.

Other units such as the metre and the second have already been defined without reference to a physical object.

For example, a metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole.

It is now defined as 1/299792458 of the distance light travels in a vacuum in one second.

A second was once worked out as a 'mean solar day' divided by 1/86,400.

It is now calculated by measuring the oscillations of an atom of the radioactive metal Caesium 133 at zero degrees.

The International Committee for Weights and Measures has already recommended that the kilogram be defined in similar terms.

However, it is yet to agree on the definitive method for doing this and there are several proposed approaches put forward by scientists.

The final definition will have to be agreed upon by scientists at the General Conference on Weights and Measures.